According to the reasons listed here, PHP isn't good for much more than just the smallish, more personal sites. It was originally written back in 2004, but has just been recently updated (April 2006) with a more current state of PHP.

I have been developing in PHP for six years now. PHP is very easy to program in. But PHP also has some serious flaws. Below I give some reasons why you have to do some serious thinking before implementing a large scale web application in PHP.

Some of the reasons they give include:

Many PHP-modules are not thread safe

Non-standard date format characters

No Unicode

It's interesting to see how many of these reasons seem to be more of a preference than a real standard, and the "crippled for commercial reasons" comments are very interesting. Also, several of these will be addressed in the next version of PHP, version 6.